After enjoying two days of consecutive gains, pre-FOMC jitters dragged the FTSE 100 0.2pc lower to 6,218.07 in mid-morning trade.

European bourses were relatively flat, as the German DAX climbed 0.2pc, while the CAC in Paris was marginally higher - up 0.04pc. Traders in the City said investors were refraining from buying up big positions before the Fed’s announcement.

Chris Beauchamp, of IG, said: “Stock markets in Europe are sliding as the bullish sentiment is still absent.”

The press conference after the interest rate decision will be carefully watched, and even if rates are kept on hold, as traders are preparing themselves for hawkish language.

“Janet Yellen is a known dove, but no central banker wants to give the market a free run and state rates will [not] stay ultra-low forever,” Mr Beauchamp added.

Engineering companies became the biggest FTSE fallers after peer Rotork issued a profit warning driven by August trading.

The mid-cap stock plunged 13pc, and is now set for its worst day in 15 years, as the company said last month was particularly weak, due to an increased number of order deferrals and cancellations.

Traders in the City said Rotork’s warning on profits kicked Smiths Group and Weir to the bottom of the FTSE 100. Smiths fell 4.4pc and Weir was 3.9pc lower.

Mike van Dulken, of Accendo Markets, said Weir was continuing to suffer from its pending demotion from the FTSE 100, which will take effect from September 21.

Meanwhile, mining shares have been hit by an earthquake off the Chilean coast. Despite Chilean mines emerging undamaged from the quake, stocks have fallen amid concerns of oversupply and persistent woes over the Chinese economy.

Rebecca O’Keeffe, of Interactive Investor, said: “Antofagasta is a pure play on copper, so the combination of lower copper prices, the temporary suspension of mining in Chile, and general uncertainty surrounding the Federal Reserve decision has seen its share price fall.”

Elsewhere, Ocado bucked the downward trend after two bullish broker notes lifted it to the top of the mid-cap index. Shares leapt 9.5pc to 350.1p in mid-morning trade after Morgan Stanley began covering the stock.

Describing it as “a disruptive long-term winner”, the bank said online grocery is the “next large opportunity for e-commerce globally”.

Deutsche also upgraded the online grocer’s rating to “hold”, as the bank believes the upside and downside risks of the possible launch of Amazon Fresh in London are evenly balanced.

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